Unfortunately, MapKit on the iPhone does not include a way to set the zoom level. Instead, the zoom level is set implicitly by defining the
MKCoordinateRegionof the map’s viewport. When initializing the region, you specify the amount of distance the map displays in the horizontal and vertical directions. The zoom level is set implicitly based on these distance values. Instead of dealing with this region business, I wrote a category that adds support for setting the zoom level of an
Troy also wrote an extensively detailed post on the mathematical background of his code. Great stuff!